Although cell-phones have become increasingly popular over the last decade, their use in “noisy” environments, such as restaurants, airports, train stations, arenas, and the like, have been limited due to a lack of appropriate speech enhancements. In the past, background noise has been addressed mainly by manual volume controls located on the mobile handset. However, these manual controls are usually inconvenient to use and ineffective in compensating for the background noise of the environment, especially in environments where the background noise changes rapidly. Manual interaction by the caller is often inaccurate both in time and magnitude, that is, compensation may come too late and too loud.
In these adverse environments, automatic noise compensation greatly improves conversation conditions and reduces the inconvenience of the caller. Referring to the two sides of the telephone link as “near-end” and “far-end,” an elementary realization of a near-end noise compensator consists of a noise-adaptive gain controller for the far-end signal, whereby the gain applied is proportionate to the near-end noise level.
Intelligibility losses due to background noise are well known. One solution to reduce the impact of background noise on intelligibility losses uses a “clipping” technique. Although clipping improves intelligibility, it adds distortion to the received signal. Other techniques have improved intelligibility by high-pass filtering, dynamic compression, or a combination of these two. However, none of these early systems used an expander to avoid undesired far-end noise.
In addition, although there has been a proliferation in the research of automatic noise compensation and automatic noise reduction, these two functions have been implemented as separate entities (i.e., without being coupled to one another). In the absence of coupling the two functions, noise compensation leads to far-end noise modulation (i.e., far-end noise is modulated by near-end noise). In other words, all the level changes of the near-end noise are echoed in the far-end noise, which is an unwanted and annoying artifact. For example, when the near-end noise follows a pattern of loud-soft-loud, the far-end noise follows the same pattern. Noise compensation should be designed such that near-end noise purposely and only modulates far-end speech, but no modulation should result for the far-end noise. In general, amplitude modulation of a near-stationary signal such as steady background noise sounds unnatural and artificial while amplitude modulation of a non-stationary signal, such as speech, is acceptable to an extent. Human ears are by a magnitude more sensitive to amplitude modulation of near stationary noise than to amplitude modulation of speech.
The previous noise compensation methods unfortunately have the unpleasant side-effect of amplifying the far-end noise, since the noise compensation methods have been unable to discriminate between the signal and noise. Therefore, whenever the signal is amplified, the far-end noise is also amplified. One method to reduce this unwanted side-effect is to use an expander operating in the time domain. The method works well if the signal is disturbed only by moderate or weak background noise. Unfortunately, if the far-end signal is disturbed by strong ambient noise, the time-domain expander is unable to adequately reduce the noise. Thus, there is a need in the art for a compander which couples the noise compensation and noise reduction functions into a single compander which is able to adequately reduce excessive far-end noise and eliminate any effects of the near-end noise echoed in the far-end noise signal.